In recent years, the development of electronic devices using organic semiconductor materials has widely been performed, and the development of organic electro-luminescence (EL) light emitting elements, organic thin film transistors (TFTs), organic solar cells, and the like have been reported. Among them, organic EL displays are expected as a promising technique closest to the practical realization thereof.
The configurations of organic EL display panels are classified into passive-matrix types and active-matrix types. The passive-matrix types are premised on impulse operation, and the current value to be flown at the time of lighting becomes large. Consequently there is a serious trade-off between the brightness of the passive-matrix type organic EL display and the span of the life thereof, and the passive-matrix type one is regarded as the one from which it is difficult to obtain a high brightness display panel. On the other hand, the active-matrix type organic EL display is not always driven by the impulse operation, and can be operated in nearly always lighted state. The active-matrix type one can consequently decrease the current value to be flown at the time of lighting, and is regarded as the one effective for the elongation of the span of the life of the organic EL element. However, the active-matrix type one has a problem of the conquest of the variations of TFTs and organic EL elements, and characteristic drifts.
Accordingly, a voltage programming method, a current programming method, and the like have been proposed, and the trials of correcting the variations of the TFTs and the characteristic drifts (chiefly threshold drifts) have been performed.
A first patent document (U.S. Pat. No. 6,229,506) discloses pixel circuits that compensate the variations of the thresholds of TFTs by a current programming method.
A second patent document (U.S. Pat. No. 6,373,454) discloses pixel circuits that perform more precise correction (the correction of the mobility changes and the like of TFTs) by a different current programming method from that of the first patent document.
A third patent document (WO-A1-2005029455) discloses an invention of correcting the characteristic drifts of TFTs and organic EL elements by flowing currents through organic EL elements by using current mirror circuits even if the saturation characteristics of TFTs are not sufficient (i.e., the TFTs cannot function as constant current sources).
A fourth patent document (Canadian Patent No. 2,472,689) discloses an invention for correcting current signals by using a current programming method and feedback circuits. FIG. 13 illustrates a comparative example of the feedback drive method disclosed in the fourth patent document. The pixel circuit of a pixel 30 includes a current mirror circuit including transistors T3 and T4, and a programming current is fed back to a circuit 32 through the reference transistor T3. At that time, the programming current is guided to the inverting terminal of an amplifier 33 in the circuit 32 through a feedback line 36. The pixel 30 further includes a select line 34, a data line 35, a holding capacitor Cs, and a light emitting element 31.
Because a signal to be fed back is the programming current itself in the circuit 32, a very small current must be fed back when low brightness is programmed. Because the addition of the feedback circuit is originally premised, the parasitic capacitance of the circuit is large, and the charging by very small current takes a long time, which is unsuitable for high speed driving.
The problem of the current programming method is that the charging of the load capacitance of a data line including the parasitic capacitance takes a long time because the current signal of low brightness is a small current, and that it is difficult for the current signal flowing through the pixel circuit in the low brightness especially to reach a steady state. Then, it is consequently hard to correctly program a current signal in the pixel circuit.
On the other hand, because a voltage signal is supplied onto a data line in the voltage programming method, the aforesaid problem of the current programming method does not exist, but it is difficult for the voltage programming method to deal with the variations of the threshold voltages and the mobility of transistors.
As described above, although the current programming method is excellent in the correction of device characteristics, the current programming method has a problem of the difficulty of high speed driving.